Formed part of sintered iron and method and sintering tile for making same

ABSTRACT

A formed part of sintered iron, and method and sintering tile for making same, according to which the formed part to be sintered is for purposes of providing the same with an austenitic wear resistant surface reacted during the sintering process with a substance from which at least at the sintering temperature austenite forming elements are diffused into the surface of the formed part.

This is a continuation of application Ser. No. 419,346, filed Nov. 27,1973, now abandoned.

The present invention relates to a formed part of sintered iron whichhas a wear resistant surface, and also concerns a method and a sinteringtile for making such formed part. The invention is applicable tosintered iron metals which means to pure iron and to iron alloys.

The sintering technique as a manufacturing process is known, whichpermits the manufacture of formed parts with so narrow tolerances thatthe finished parts are as a rule obtained in a condition ready forinstallment. The conditions of use of sintered formed parts frequentlyrequire a wear resistant surface in order to increase their life span orstaying time. Heretofore, for purposes of creating wear resistantsurface layers, the case hardening, nitriding, carbon nitriding, and inless frequent instances inductive surface hardening have become known.The wear resistance which results from employing this method is based ona change in the structure of the surface layer. As a result of thehardening method, there is formed for instance martensite, the nitridingmethod brings about the formation of iron nitride. Finally, also partsof carbon containing sinter steel can be hardened through while inaddition to the surface also the core of the workpiece becomes wearresistant. A further possibility consists in galvanically depositingwear resistant layers for instance hard chromium layers. All of thesemethods, however, require that in addition to the working operationsnecessary for the manufacture of sintered iron parts, additional workingoperations have to be carried out for obtaining the wear resistantsurface layer. To this end, additional devices are necessary so that thecosts of a sintered part are further increased in an undesired manner byone of the abovementioned post treatments.

A further drawback of the hardening methods referred to above consistsin that the formed part may during the heat treatment for producing thewear resistant surface get warped or distorted so that the narrowtolerances which represent a particular advantage of the sintered metalformed parts are again increased. The sintered parts heat treated inthis manner therefore have frequently to be subjected to a posttreatment by chip removing which causes additional costs. With thesintered part hardened in customary manner, it is also highlydisadvantageous that these sintered parts due to the high strength ofthe marginal layers cannot be roll pass designed and therefore in mostinstances have to be post treated by a chip removing operation.

It is, therefore, an object of the present invention to provide sinteredformed parts, and a method of making same which will overcome the aboveoutlined drawbacks. This object and other objects and advantages of theinvention will appear more clearly from the following specification inconnection with the accompanying drawing diagrammatically illustrating asection through a sintering tile for carrying out the method accordingto the invention.

The formed part of sintered iron according to the invention with wearresistant surface is characterized primarily by a surface layer of anaustenitic structure.

Such austenitic surface layers the characterizing structure component ofwhich consists of cubic surface sintered iron, so-called austenite, arehighly wear resistant. The method according to the invention has thegreat advantage that the high final hardness can be obtained bycalibrating or by roll designing.

Austenitic surface layers can be obtained with nickel or copper or withmanganese alone or in combination. Preferably, the surface layer has acomposition which corresponds to that of an austenitic manganese hardsteel. The generation of an austenitic layer with manganese isadvantageous insofar as the other mentioned elements are more expensivethan manganese. Furthermore, it is advantageous when theaustenite-surface layer is cold work hardened. Austenite as such hasalready a certain hardness. The hardness may, however, be greatlyincreased by cold work hardening so that the surface layer becomes wearresistant. The cold work hardening can be effected by the customaryhardening of a formed part. This property is known, when making machineparts of manganese hard steel, as work hardening. With a formed part ofsinter iron according to the present invention, only the surface layerconsists of an austenitic steel, whereas the core of the formed partconsists of the customary sinter iron alloy. The surface layer is madepreferably by diffusing the austenite forming alloy elements into thesurface of the formed part. Diffusion is the most simple possibility ofentering into the surface of the formed part those elements which arenecessary for the formation of austenite. The method according to theinvention for making a formed part of sinter iron is characterizeprimarily in that the formed part, for pusposes of creating anaustenitic wear resistant surface layer, is during a sintering processbrought into reaction with a substance from which at least at the sintertemperatures, austenite forming elements diffuse into the surface of theformed part. Therefore, due to the method according to the invention,there is, (by diffusion on sintered formed parts of iron and ironalloys) generated air austenitic surface layer which is characterizedprimarily by its cold hardening ability.

The advantage of the method according to the invention is seen in thatwhen practicing said method, no additional devices are necessary otherthan those which are anyhow present when making sinter iron. No additionheat treatment is necessary. The surface layer is created during thesintering operation which means when inert gas annealing at temperaturespreferably between 1,000° and 1,300° C. Each sintering iron formed parthas anyhow to go through this sintering process. Consequently, themethod according to the invention is considerably less expensive thanany heretofore known treating and hardening method. A further advantageis seen in the fact that the method according to the invention can bepracticed in any customary sintering atmosphere, in a vacuum and, whenemploying completely tightly closed sintering tiles, even in air andother atmospheres. It is merely necessary to see to it that the materialof the tile will not be attacked by the respective atmosphere. Whencarrying out the method in air, it is suggested that the tile consistsof high heat resistant steel. In order to provide sintered iron partswith a wear resistant surface layer of manganese steel, a substance isnecessary from which it is possible to diffuse manganese and carbon intothe formed part. As such substance a substance has been foundparticularly advantageous which consists of the following example:

from 0.1 to 10% of a carburizing agent;

from 0.1 to 25% of thermally disassociating carbonate;

from 0.1 to 30% of halogen salts;

from 1.0 to 20% of silicon oxide or aluminum oxide;

remainder ferromanganese.

Under certain circumstances, the carbonizing agent need not be employedif ferromanganese is used with sufficient carbon content. The substancewill then be a mixture for example of:

from 0.1 to 25% of thermally disassociating carbonate;

from 0.1 to 30% of halogen salts;

from 0.1 to 20% of silicon or aluminum oxide;

remainder ferromanganese with from 5 to 10% carbon.

As carbonizing agents there may be used active carbon, charcoal, bonecoal, coke powder and any other carbonizing agents customary in the art.Active carbon, however, is preferred particularly in view of its purityand its large surface. As thermally disassociating carbonate, bariumcarbonate, as well as all other carbonates are suitable which at thesintering temperature will thermally disassociate in carbon dioxide andother molecule residues.

As hallogen salts there may be used principally all chlorides which atthe sintering temperature thermally disassociate in chlorine ion and amolecule residue. The chlorides may also be replaced by other hallogensalts for instance fluorides, bromides or iodides. However, ammoniumsalts, especially ammonium chloride are preferred inasmuch as they havethe advantage of disintegrating in a halogen residue (chlorine,fluorine, bromine or iodine) and ammonia. As a result thereof,additionally a nitrogen enrichment (Aufstickung) of the treated formedparts takes place. The nitrogen enrichment is advantageous in as much asdue to the nitrogen, the austenite is additionally stabilized.

The aluminum oxide or silicon oxide serve as indifferent substances forpreventing a sintering together of the mixture which gives off theaustenite forming elements. Fundamentally, to this end any substance maybe employed which has this effect.

It is advantageous to use as austenite forming substance a powder with agranular size of from 0 - 5mm, preferably from 0 - 0.6mm so that as faras possible a maximum active surface is obtained from which in responseto the heating of the substance to sintering temperature of the formedpart, the austenite forming elements can separate.

As an example a substance has proved highly satisfactory for practicalpurposes, which consists of:

from 4 to 6% active carbon;

from 5 to 8% of barium carbonate;

from 10 to 20% of ammonium chloride;

from 4 to 8% of aluminum oxide;

and the remainder ferromanganese.

Into this mixture or the substance which gives off the austenite formingelements, there are packed the powder pressed, in other words not yetsintered parts to be formed and are then subjected to the customarysintering process. During the sintering process there willsimultaneously be formed the desired austenitic surface layer withoutthe necessity of taking additional steps. The surface of the formedparts will, however, be somewhat rougher than customary for sinteredparts because the mixture has the inclination to stick. When the partsare fully surrounded by a powder mixture, also the evaporation oflubricants, for instance of stearates, will be made somewhat moredifficult which are usually added to sinter powders for facilitating theform pressing. In instances in which this limitation is, however, notobjectionable, the packing of the form parts into the active mixturerepresents the most simple and least expensive method of making formedparts according to the invention.

If, however, the above mentioned limitations should be objectionable,the sintering process has to be carried out in such a way that theformed parts will not come into direct contact with the mixture. To thisend, a closed reaction chamber is necessary in which the austeniteforming elements diffuse from the mixture through the gaseous phase intothe formed part. This may occur for instance in a sinter tile as it isdiagrammatically shown in the drawing.

With reference to the drawing, a glow tile or sinter tile 1 contains thepowder mixture 2 which gives off the manganese and carbon, said mixture2 being spread at the bottom of the tile 1.

Furthermore, provided at the bottom of the tile are spacer members inthe form of ribs 3, the tips of which extend beyond the powder mixture2. Located on said ribs 3 are the pressed bodies 4 from which bysintering the form parts are formed. The sinter tile 1 is closed by acover 5 which is sealed by any thermally resistant mass 6. The escape ofmanganese and carbon from the active mixture as well as the diffusing ofthe manganese and of the carbon into the pressed bodies 4 is in thisinstance effected through the gas phase during the sintering operation.

The sintering temperature is the customary temperature of from 1,000° to1,300° C customary in connection with the manufacture of sintered ironparts. Also the sintering time corresponds to the customary values offrom about 20 minutes to 1 hour. The thus treated formed parts arecharacterized by a smooth surface customary for sintered carbon parts,and for the wear resistant surface layer according to the invention withan austenite structure.

Following this working operation, the surface hardness of the formedparts amounts to approximately HV=200 . . . 300. The hardness desiredfor the required wear resistance can be imparted upon the dormed partsby subjecting them to use (work hardening). Sinter formed parts are,however, generally calibrated after the sintering operation forobtaining better tolerances. Inasmuch as the calibration brings about acold deformation in the surface, it will be appreciated that as a resultthereof with formed parts according to the invention which during thepreceding sintering operation have obtained an austenitic surface layer,a considerable increase in hardness if effected up to HV=600-700. Alsoin this instance an additional working operation is not necessarybecause the major portion of all sinter formed parts anyhow is subjectedto a calibration. The calibration brings about a partial conversion ofthe austenite to martensite.

Practical tests have proved that by means of the method according to theinvention, contour true austenitic surface layers are obtained. Thechemical composition corresponds approximately to the Hatfield manganesehard steel. The analysis resulted in 0.6 . . . 1.8% C and 10 . . . 15%Mn.

It is, of course, to be understood that the method according to thepresent invention is, by no means, limited to so-called greenworkpieces, i.e. not yet sintered workpieces, but is also applicable toprovide formed parts, which have already been sintered in conformitywith the heretofore customary methods, with an austenitic surface layer.In such an instance, a second sintering process in conformity with thepresent invention is necessary which may then be followed by acalibrating process.

It is also to be understood that the present invention is not limited tothe specific method set forth in connection with the drawing but alsocomprises any modifications within the scope of the appended claims.

What we claim is:
 1. A part formed of sintered iron body having awear-resistant surface layer of an austenitic steel which has manganeseadded to the material of said layer, so that said surface layer has agreater proportion of manganese than remainder of the body of said part,said surface layer constituting exclusively a manganese diffusion alloypurely of austenitic steel and also being cold deformed for greaterhardness than said body.
 2. A formed part as claimed in claim 1, inwhich said surface layer has a greater proportion of carbon than theremainder of said body.
 3. A formed part as claimed in claim 1, in whichsaid surface layer has increased hardness deformation produced by workhardening.